Zutara is awesome, pure and simple.
It crosses genres. You do any sort of fanfiction with Zutara, with any sort of demographic.
With Kataang, it's just the fangirl. All it is is fuff and boring love-dovey bull.
That kind of thing doesn't exist in real life, cept among pre-teens. And we all know how long the pre-teen relationship lasts.
Zutara is more mature, more thoughtful, and a hell of alot smarter than Kataang.